1. Field of the Invention
The present invention relates to a technique for embedding digital-watermark information in image data.
2. Related Background Art
In recent years, computers and networks have advanced remarkably, and various kinds of data such as character data, image data, audio data, and the like are handled in computers and networks. Since such data are digital data, copies of data with equivalent quality can be readily created in the current environment.
For this reason, in order to prevent illicit copies and interpolation of data, and protect the copyright, copyright information and user information is embedded in image data, audio data, or the like as digital-watermark information. This is a technique for secretly embedding information in data by performing predetermined processing of this data.
Hence, since the copyright information or user information can be obtained by extracting the digital-watermark information from data, an illicit copy can be tracked.
As a method of embedding digital-watermark information, there are two methods, i.e., a method of embedding information in the space domain, and a method of embedding information in the frequency domain.
As an example of the method of embedding information in the space domain, a scheme of IBM (W. Bender, D. Gruhl, and N. Morimoto, xe2x80x9cTechniques for Data Hidingxe2x80x9d, Proceeding of the SPIE, San Jose Calif., USA, February 1955) and the like are known as one based on patchwork.
On the other hand, as an example of the method of embedding information in the frequency domain, a scheme of NTT (Nakamura, Ogawa, and Takashima, xe2x80x9cMethod of Watermarking under Frequency Domain for Protecting Copyright of Digital Imagexe2x80x9d, SCIS ""97-26A, January 1997) is known as one using the discrete cosine transform. On the other hand, a scheme of National Defense Academy of Japan (Onishi, Oka, and Matsui, xe2x80x9cWatermarking Scheme to Image by PN Sequencexe2x80x9d, SCIS ""97-26B, January 1997) is known as one using the discrete Fourier transform. Furthermore, a scheme of Mitsubishi and Kyushu University (Ishizuka, Sakai, and Sakurai, xe2x80x9cOn Experimental Evaluation of Steganography with Wavelet Transformxe2x80x9d, SCIS ""97-26DP, January 1997), and a scheme of Matsushita (Inoue, Miyazaki, Yamamoto, and Katsura, xe2x80x9cDigital Watermark Technique Based on Wavelet Transform and its Robustness on Image Compression and Transformationxe2x80x9d, SCIS ""98-3.2.A, January 1998) are known as techniques using the discrete wavelet transform.
Note that requirements for the digital-watermark technique include:
(1) no visible image quality deterioration of original image;
(2) superior robustness against attacks; and
(3) large size of information that can be embedded.
Attacks will be briefly explained below.
Attacks are acts of deleting or destroying watermark information, and there are two types of attacks. One type is deliberate attacks, which include an attack of appending noise to image data embedded with watermark information, an attack of deleting an area where watermark information is likely to be embedded, and the like. The other type is attacks by means of image processes, such as image compression, enlargement and reduction, cut-out, gradation conversion, print-out, scanning, and the like for image data embedded with digital-watermark information. Hence, watermark information embedded in image data must be robust against any of these attacks.
The purposes of embedding digital-watermark information in digital contents such as image data and the like are to protect the copyright of digital contents, and to detect interpolation of digital contents. In case of the former purpose, digital-watermark information which is highly robust against attacks is preferably embedded in digital contents. In case of the latter purpose, digital-watermark information which is less robust against attacks is preferably embedded in digital contents.
Conventionally, when digital-watermark information which is highly robust against attacks is embedded in an image, the image quality visually deteriorates compared to an original image.
When an image embedded with digital-watermark information has been attacked, it is hard to reliably extract the digital-watermark information.
The present invention has been made to solve the above problems, and has as its object to provide a technique for embedding digital-watermark information in image data while minimizing deterioration of original image quality.
In order to achieve the above object, according to a preferred embodiment of the present invention, there is provided a data processing apparatus comprising:
an input unit for inputting image data composed of a plurality of coefficients; and
an embedding unit for embedding digital-watermark information in a coefficient having a value within a predetermined range of the input image data.
Also, there is provided a data processing apparatus comprising:
an input unit for inputting image data composed of a plurality of coefficients; and
an embedding unit for detecting a coefficient having a maximum absolute value of the input image data, and embedding digital-watermark information in the detected coefficient.
Furthermore, there is provided a data processing apparatus comprising:
an input unit for inputting image data composed of a plurality of coefficients; and
an embedding unit for embedding digital-watermark information at a first strength in a coefficient having a value within a first range, and embedding digital-watermark information at a second strength in a coefficient having a value within a second range.
Other objects and features of the present invention will become apparent from the following description of the embodiments taken in conjunction with the accompanying drawings.